The present invention relates to a superconducting quantum interference device (hereinafter abbreviated as SQUID) for detecting a very small magnetic field, current, voltage, electromagnetic wave or the like, and more particularly, to a structure of a detecting coil of a SQUID flux meter having a high spatial resolution.
FIG. 9 is a structural drawing of a prior art SQUID. A superconducting ring comprising Josephson junctions 1, a washer coil 2 and a detecting coil 3 is formed on a substrate 6. In FIG. 9, the Josephson junction 1 is denoted simply by a symbol. A feedback-modulation coil 4 for FLL (Flux Locked Loop)-driving the SQUID is magnetically coupled to the washer coil 2. Magnetic flux detected by the detecting coil 3 is directly connected with the superconducting ring. This SQUID is an integrated SQUID in which all the components such as the Josephson junctions 1 and the detecting coil 3 are formed on the substrate 6 by a superconductive thin film. The superconductive thin film composing the SQUID is a Nb thin film, and a Nb/Al--AlOx/Nb tunnel junction is used for the Josephson junction.
FIG. 10 shows a scanning SQUID microscope constructed with the SQUID shown in FIG. 9. A sample 12 is attached to a sample holder 13. The SQUID 10 is mounted on a SQUID holder 14 and the surface of the sample 12 is scanned by an XYZ scanning controller 15. The SQUID 10, the sample 12, the sample holder 13 and the SQUID holder 14 are disposed in a vacuum chamber 17 within a cryostat 11. The vacuum chamber 17 is cooled by liquid helium 16, i.e. coolant.
The scanning SQUID microscope measures a distribution of magnetic field in an area on the order of micron geometry on the surface of the sample. It is necessary to miniaturize the detecting coil 3 in order to enhance the spatial resolution of the SQUID microscope. This, the SQUID 10 is realized so as to have a compact structure in which the detecting coil is integrated and formed by the superconductive thin film. It is also necessary to form the detecting coil 3 in close proximity to the sample 12.
It is also necessary to vary the temperature of the sample in the SQUID microscope. For instance, there is a case which the sample is measured by setting the temperature of the sample at a temperature higher than a critical temperature of the superconductor composing the SQUID. However, the temperature around the detecting coil 3 disposed in close proximity to the sample 12 exceeds the critical temperature of the detecting coil in the prior art SQUID. In such case, the detecting coil cannot maintain a superconducting state and cannot detect magnetic flux.
Thus, it is difficult to vary the temperature of the sample while enhancing the spatial resolution of the SQUID microscope.
Because the Josephson junction section and the detecting coil are made of the same superconductive thin film in the prior art SQUID, the critical temperature of the Josephson junction section is equal to the critical temperature of the detecting coil. When the SQUID microscope is constructed by using the above described SQUID and the sample is measured when the temperature of the sample is higher than the critical temperature of the SQUID, there is a case when the temperature around the detecting coil which is disposed in close proximity to the sample exceeds the critical temperature. Then, in such case the detecting coil is unable to maintain the superconducting state and to detect magnetic flux.